The invention relates to adhesive compositions comprising epoxy resins and more particularly to such adhesive compositions used as underfill adhesives for integrated circuits.
Early integrated circuit packaging involved wire bonding for connecting the integrated circuit to the printed circuit board. One alternative to wire bonding is solder bump interconnections. This method of interconnection is increasing in usage due to improved performance and increasing Input/Output (I/O) density. Flip chip bonding using solder bumped chips has allowed the use of area arrays on chips.
In flip chip bonding using solder bumped chips, a solder paste (flux) is applied to the circuit board, the chip pads are aligned with the traces on the substrate, and then the assembly is heated in a reflow oven. During this heating, the solder melts and a metallurgical bond between the chip and the substrate is formed. The surface tension of the solder during melting also leads to self-alignment of the pad pairs. After this reflow process, the solder flux must be washed away to remove corrosive residue. The assembly must be dried after this washing step.
The electronic assembly then requires additional environmental protection. In most cases, the underside of the assembly is encapsulated using an epoxy adhesive containing an inorganic filler. This adhesive is applied by allowing capillary forces to pull the resin underneath the chip. The filler, typically silica, is added to reduce the coefficient of thermal expansion of the underfill resin.
As chip sizes and the number of solder bumps on them increase, the method of adding an underfill material to the package using capillary forces will become less effective. An alternative to the capillary method of underfilling is to pre-apply, to the substrate, an adhesive that has fluxing properties. The adhesive, after fluxing the solder and allowing interconnection to occur, cures and becomes the underfill.
However, fluxing adhesives that contain liquid or easily volatilized anhydrides for example, provide bondlines that contain voids after cure. These voids can lead to premature solder fatigue failure in underfill applications. Fluxing adhesives that contain fluxing crosslinking agents can have poor shelf life or premature gelation or both, inhibiting solder flow.
In one aspect, the invention provides a one-part, thermally curable, adhesive composition comprising epoxy resin substantially free of hydroxyl functionality; anhydride curing agent, wherein the anhydride has a weight loss of less than 10 percent as determined by thermogravimetric analysis wherein the temperature is ramped from ambient to 140xc2x0 C. at a rate of 90xc2x0 C./minute, held isothermal for 1 minute, then ramped to a temperature of 225xc2x0 C. at a rate of 90xc2x0 C./minute and then held isothermal for 2 minutes; hydroxyl containing compound that is substantially insoluble in the epoxy resin at a temperature of less than 80xc2x0 C.; and optionally, catalyst. A preferred use of the adhesive composition is as a fluxing adhesive composition.
In another aspect, the invention provides an electrical component assembly comprising an electrical component having a plurality of electrical terminations, each termination including a solder bump; a component carrying substrate having a plurality of electrical terminations corresponding to the terminations of the electrical component; and an adhesive disposed between and bonding the electrical component and the substrate together, the solder bumps being reflowed and electrically connecting the electrical component to the substrate, the adhesive comprising the reaction product of an adhesive composition of the invention.
In another aspect, the invention provides a method of bonding an electrical component assembly comprising the steps of providing an electrical component having a plurality of electrical terminations, each termination including a solder bump; providing a component carrying substrate having a plurality of electrical terminations corresponding to the terminations of the electrical component; providing a sufficient amount of an adhesive composition of the invention to bond the electrical component and the component carrying substrate together onto the substrate; contacting the electrical component with the adhesive composition; and curing the adhesive composition.
In yet another aspect, the invention provides a thermally-curable one-part adhesive composition comprising an epoxy resin substantially free of hydroxyl functionality and an anhydride curing agent. The adhesive compositions of this aspect are stable in that they are expected to have a relatively long shelf-life as compared to other epoxy resin/anhydride compositions. The adhesive compositions of this aspect have utility bonding substrates where a fluxing agent is not necessary.
xe2x80x9cSubstantially free of hydroxyl functionalityxe2x80x9d means the epoxide equivalent weight is at or near the theoretical epoxide equivalent weight (that is, within 5 percent or less of the theoretical epoxide weight) and there is no hydroxyl group inherent in the monomeric form of the epoxide.
xe2x80x9cSubstantially insolublexe2x80x9d means that when a particulate form (1-10 mil (0.025-0.25 mm)) of the insoluble component is added to a liquid component, an opaque blend is formed which remains unchanged and only goes translucent upon heating of the mixture to a temperature of 80xc2x0 C. or greater.
xe2x80x9cFluxing agentxe2x80x9d means a material that cleans a metal, for example solder, surfaces of oxides.
xe2x80x9cAdhesivexe2x80x9d means a cured adhesive composition.
xe2x80x9cParts per hundredxe2x80x9d means parts per 100 parts by weight of the total amount of epoxy resin, anhydride curing agent, hydroxyl containing compound, and catalyst.
Advantages of the adhesives and adhesive compositions of the invention include processing stability (as measured by gel time), a shelf life of greater than 4 weeks at ambient temperature under a nitrogen atmosphere, a pot life of greater than 8 hours at 80xc2x0 C. (defined as a doubling of viscosity), minimal outgassing during cure (as measured by thermogravimetric analysis), and high fluxing activity (as measured by solder spread). Additionally, the adhesives and adhesive compositions of the invention do not substantially interfere with the surface tension/self-alignment feature of the solder. It has also been observed that only minimal force for short periods of time is required during placement to provide constructions that yield metallurgically and electrically bonded component during a reflow process with no additional added pressure required.
The preferred adhesive compositions and resulting adhesives of the invention provide a balance of fluxing properties and improved potlife. This balance is achieved by using a combination of purified epoxy resins, less volatile, relatively high molecular weight anhydrides, and hydroxyl containing compounds that are substantially insoluble in the epoxy resin-anhydride mixture at temperatures less than about 80xc2x0 C. The adhesive compositions of the invention improve potlife and maintain fluxing capability by generating a fluxing agent just prior to the melting point of the solder. The fluxing agent is generated by the reaction of the hydroxyl containing compound (which becomes soluble at an elevated temperature) and the anhydride, neither of which provide fluxing individually. The purified epoxy resins prevent premature gellation and the substantially insoluble hydroxyl containing compound provide improved potlife since the hydroxyl containing compound reacts with the anhydride at temperatures of about 80xc2x0 C. and above. Anhydrides having low volatility prevent void formation in the adhesive bondline during cure.
The adhesives and adhesive compositions of the invention do not include polyimide oligomers having a molecular weight of up to about 8,000 g/mol (Mn) having a backbone that is unreactive with an epoxy resin as described in U.S. Ser. No. 09/611,450, entitled Polyimide Hybrid Adhesives, filed on Jul. 6, 2000, now U.S. Pat. No. 6,294,259.
The adhesives and adhesive compositions of the invention contain one or more epoxy resins. Useful epoxy resins include, for example, substituted or unsubstituted aliphatic, cycloaliphatic, aromatic and/or heterocyclic polyepoxides, such as glycidyl esters, glycidyl ethers, glycidyl-functional amino phenols, glycidyl amines, or epoxidized olefins, and combinations thereof.
Specific examples of epoxy resins useful in the adhesives and adhesive compositions of the present invention include, but are not limited to, bisphenol A-epichlorohydrin epoxy resins, bisphenol F-epichlorohydrin epoxy resins, aliphatic mono glycidyl ethers, aliphatic diglycidyl ethers, diglycidyl-functional amino phenols, aliphatic multifunctional glycidyl ethers, and aliphatic glycidyl esters.
Examples of useful bisphenol A-epichlorohydrin epoxy resins include, but are not limited to, EPON(trademark) Resins 825, 826, and 828, available from Shell Chemical Company, Houston, Tex.; D.E.R.(trademark) 330, 331, and 332, available from Dow Chemical Company, Midland, Mich.; and ARALDITE(trademark) GY 6008, GY 6010, and GY 2600, available from Vantico Inc., Brewster, N.Y.
Examples of useful bisphenol F-epichlorohydrin epoxy resins include, but are not limited to, EPON(trademark) Resin 862, available from Shell Chemical Company, Houston, Tex.; and ARALDITE(trademark) GY 281, GY 282, GY 285, PY 306, and PY 307, available from Vantico Inc., Brewster, N.Y.
Examples of useful mono, di and multifunctional glycidyl ether resins include, but are not limited to, XB 4122, TACTIX 556, TACTIX 742, and ARALDITE 510, available from Vantico Inc., Brewster, N.Y; and EPON(trademark) 1510, HELOXY(trademark) Modifier 107, and HELOXY(trademark) Modifier 48, available from Shell Chemical Company, Houston, Tex.
The epoxy resins are preferably ionically clean in that they are substantially free of ionic species. The epoxy resins are also preferably substantially free of hydroxyl functionality. The epoxy resins may also contain polymeric and/or glass microspheres as described in U.S. Ser. No. 09/402,336, now U.S. Pat. No. 6,288,170, incorporated by reference herein for cured epoxy resins that are removable by the application of heat and methods of making same.
Removal of residual ionic halogens can be accomplished by first reacting the epoxy resin with a base. The base is present in an amount which exceeds the molar equivalent based on the materials comprising hydrolyzable halide. This amount depends on the starting epoxy resin. For example, if no other acids are present, a theoretical amount of base can be used based on the ppm of hydrolyzable halide. In other situations, for example, 100 percent to 200 percent base is required.
The epoxy resin may be combined with a base at room temperature to form a mixture or in other situations, the epoxy resin may be pre-heated. Thus, the heating and agitation step may occur prior to and during the reaction with the base, simultaneously with the base treatment step, or after the base is added to the epoxy resin. This order is dictated by the starting epoxy resin.
The selection of the base depends upon the starting epoxy resin. Examples of suitable bases useful in the process of the present invention include, but are not limited to, hydroxides such as potassium hydroxide in water, sodium hydroxide, and lithium hydroxide, hydrides such as lithium hydride, sodium hydride (optionally in mineral oil), and potassium hydride, alkoxides such as primary, secondary, and tertiary (e.g., potassium t-butoxide in tetrahydrofuran (THF)) alkoxides such as sodium ethoxide, carbonates such as potassium carbonate and sodium carbonate, and quaternary ammonium salts.
Generally, the base strength and the temperature are such that the halohydrin closes to the epoxy and under which the epoxy does not polymerize. For example, in one case for an epichlorohydrin-derived epoxy resin, potassium t-butoxide in THF was suitable at 25xc2x0 C., but the resin polymerized at 70xc2x0 C.
The use of non-nucleophilic bases such as sodium hydride are believed to have the advantageous effect of closing the halohydrin without reacting appreciably with other base (hydrolytically) sensitive functionality such as esters. Without being bound by theory, the following is believed to occur: 
If a non-nucleophilic base is used, the process of the present invention preferably comprises the following steps: (a) distilling an epoxy resin comprising materials containing hydrolyzable halide using molecular distillation to yield an epoxy distillate; and (b) reacting said epoxy distillate with a base wherein said base is present in a quantity which exceeds the molar equivalent based on the materials containing hydrolyzable halide.
The initial distillation step removes moisture along with high molecular weight materials containing hydroxyl functionality. The product can either be neutralized with water and carbon dioxide to remove residual sodium hydride before distillation or can be distilled directly without neutralization.
The mixture is heated to a temperature suitable for reaction of the halohydrin to form the epoxy while agitated. For example, the mixture may be heated using a heat mantel. Generally, the mixture is heated between 20xc2x0 C. to 200xc2x0 C. for 1 minute to 12 hours. However, the temperature and time depend upon the starting epoxy resin, base strength and solubility, the catalytic activity of the base towards epoxy polymerization, and commercial viability.
This heating and mixing can occur after the epoxy resin and base are combined, prior to and during the base treatment step, or simultaneously with the addition of the base and base treatment step.
The mixture is usually heated to alter the viscosity which in turn helps the dispersion of the base.
The heated mixture is then neutralized, if required, using carbon dioxide to form a crude product. With the hydrides, this neutralization step may not be required. Optionally, at this point, residual salts may be removed from the crude product by filtration.
Next, the crude product is isolated by molecular distillation to yield the product. For example, a rolled film evaporator or wipe film evaporator may be used. With a rolled film evaporator, the crude product is distributed across a vertical heated surface by an efficient, self-cleaning roller wiper system into a uniform thin film. The evaporated material travels a short distance to an internal condenser. A smaller vacuum is used with low operating temperatures. (See UIC Inc., xe2x80x9cShort Path Vacuum Distillation from Laboratory to Productionxe2x80x9d, 1997). With a wipe film evaporator, a wiper is used instead of the self-cleaning roller wiper.
The distillation conditions depend on the boiling point of the crude product.
Noncondensible materials which may be the starting materials, that is, the epoxy resin, are removed during molecular distillation.
The yielded epoxy product has low levels of hydrolyzable halide, that is, from 1 to 100 ppm, preferably less than 10 ppm, more preferably less than 1 ppm.
The yielded product is preferably free of high molecular weight materials. High molecular weight material-free is defined herein as having no dimers and materials having higher molecular weight than the dimer. The epoxide equivalent weight is at or near the theoretical epoxide equivalent weight (that is, within 2 percent, preferably within 1 percent of the theoretical epoxide equivalent weight) and liquid chromatography of the distillant indicates greater than 98 percent monomeric epoxy. These data indicate an epoxy resin that is substantially free of hydroxyl functionality.
The purified epoxy product has a higher cured glass transition temperature than the less pure version which is advantageous. The purified epoxy product is also more predictable due to product consistency. The viscosity is lower than the less pure version of the same epoxy resin. There is no residual base in the epoxy product which is advantageous. Residual base may inhibit cationically cured epoxies. Other low hydrolyzable halide epoxy resins such as EPON(trademark) Resin 1462, available from Shell Chemical Company, which have some residual base are described as having a yellow color (a Gardner Color Scale value of less than 3). The purified epoxy product is colorless. For example, using the Gardner test (ASTM D1544-80), the Gardner Color Scale value is less than 0.1 for EPON(trademark) Resin 828.
The above epoxy purification process can be batch or continuous.
A preferred method of making ionically clean epoxy resins is described in U.S. application Ser. No. 09/454,558, entitled xe2x80x9cProcess for the Elimination of Materials Containing Hydrolyzable Halides and Other High Molecular Weight Materials from Epihalohydrin Derived Epoxy Resinsxe2x80x9d, filed on Dec. 7, 1999, incorporated by reference herein.
Epoxy resin is present in the adhesives of the invention at a level of from about 10 to 90, preferably from about 20 to about 80, and more preferably from about 30 to about 70 parts per hundred.
The adhesives of the invention contain one or more anhydride curing agents. As used herein, xe2x80x9canhydridexe2x80x9d also contemplates mono-, di- and poly-anhydrides. In one embodiment of the present invention, the anhydride functions as a reactant or crosslinking agent for the epoxy resin and also reacts with the hydroxyl containing compound (described below) to form an acid in-situ which functions as a fluxing agent. Useful anhydrides of the invention have low volatility as determined by thermogravimetric analysis using a procedure where the temperature is ramped from ambient to 140xc2x0 C. at a rate of 90xc2x0 C./minute, held isothermal for 1 minute, then ramped to a temperature of 225xc2x0 C. at a rate of 90xc2x0 C./minute and then held isothermal for 2 minutes, each temperature ramp in air. The anhydride, under these conditions, preferably shows a weight loss of less than 10 percent, more preferably less than 5 percent. Useful anhydrides are also capable of being dissolved into the epoxy resin. xe2x80x9cDissolved in the epoxy resinxe2x80x9d means that the blend is translucent after stirring (and heating generally to about 120xc2x0 C. to about 180xc2x0 C. depending upon the epoxy resin and anhydride used and the mix ratio). Any aromatic or aliphatic anhydride having low volatility and the solubility described above are useful in the present invention. Specific useful anhydrides include 5-(2,5-dioxotetrahydrofuryl)-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride, 5,5xe2x80x2-(1,1,3,3-tetramethyl-1,3-disiloxanediyl)bis[hexahydro-4,7-methanoisobenzofuran-1,3-dione], and combinations thereof.
In the case of an adhesive composition of the invention containing epoxy resin and anhydride and in applications where the volatility of the anhydride is not detrimental to the performance of the adhesive, the anhydride may be liquid and more volatile that the anhydrides described above. Examples of such more volatile anhydrides include methyl-5-norbornene-2,3-dicarboxylic anhydride methylcyclohexene-1,2-dicarboxylic anhydride, methyltetrahydrophthalic anhydride, and combinations thereof. The anhydride is present in the adhesives of the invention at a level of from about 5 to about 80, preferably from about 15 to about 60, and more preferably from about 20 to about 50 parts per hundred.
The adhesives of the invention may contain one or more hydroxyl containing compounds that are substantially insoluble in the epoxy resin at a temperature of less than 80xc2x0 C. Upon heating of the adhesive composition to a temperature of about 80xc2x0 C. or greater, the hydroxyl containing compound will dissolve into the epoxy resin and react with the anhydride to form an acid moiety that functions as a fluxing agent. Examples of classes of hydroxyl containing compounds that may be used in the adhesive compositions of the invention include mono-, di-, tri- and poly-alcohols and phenols including bisphenols and combinations thereof with di-functional hydroxy containing compounds being preferred. Useful hydroxyl containing compounds are substantially insoluble in the blend of the epoxy resin and anhydride at temperatures from ambient to less than 80xc2x0 C., and preferably have a weight loss of less than 30 percent when tested according to the thermogravimetric analysis described above. Specific examples of hydroxyl containing compounds that are useful in the present invention include ethoxylated bisphenol fluorene, hydrogenated bisphenol A, bisphenol Z, bis(2-hydroxyethyl)terephthalate, and 1,12-dodecandiol. Hydroxyl containing compound is present in the adhesive compositions of the invention at a level of from about 1 to about 50, preferably from about 3 to about 30, and more preferably from about 5 to about 15 parts per hundred.
The adhesives of the invention optionally, but preferably, contain one or more catalysts. The function of the catalysts in the adhesives of the invention is to accelerate the reaction between epoxy and anhydride and between the reaction product of the hydroxyl containing compound and the anhydride. Useful catalysts are latent under ambient conditions but are activated to accelerate reactions when heated above a temperature of 80xc2x0 C. or greater. Classes of useful catalysts include transition metal complexes and organic bases, such as organophosphorous compounds, and amines having the above characteristics are known to those having ordinary skill in the art. Specific examples of useful catalysts include cobalt naphthenate, cobalt (II) acetylacetonate, cobalt (III) acetylacetonate, and copper benzoate. Catalyst is present in the adhesive compositions of the invention at a level of from about 0.05 to about 10, preferably from about 0.3 to about 5, more preferably from about 0.5 to about 2 parts per hundred.
The adhesive compositions of the invention may contain additional additives that are known to those skilled in the art. Such classes of additives include but are not limited to fillers such as silica; glass and polymeric microballoons; expandable polymeric microballoons; coupling agents, for example, silane coupling agents; pigments; thixotropic agents; toughening agents; cure indicating materials; and combinations thereof. Additives are present in the compositions of the invention at a level to effect the desired result.
Addition of a silane coupling agent is optional in the preparation of cured compositions of the invention. Preferably the silane coupling agent is added to the adhesive composition when the substrate surface is glass, and oxide or any other surface that would benefit from the addition of a silane coupling agent. When present, a silane coupling agent contains a functional group that can react with the epoxy resin, e.g., 3-glycidoxypropyltrimethoxysilane.
Generally, the epoxy resin and anhydride are mixed together with stirring, preferably under an inert atmosphere, with heat until the anhydride is dissolved. The temperature at which the mixture is heated is dependent upon the structure and mix ratio of the epoxy resin and the anhydride and generally ranges from about 120xc2x0 C. to about 180xc2x0 C. for solid anhydrides. However, in the case of a liquid and non-volatile anhydride, perhaps no additional heating would be necessary. After the epoxy resin and anhydride are blended to form a non-opaque mixture and cooled to about 80xc2x0 C. or below, the hydroxyl containing compound and catalyst are blended into the epoxy resin-anhydride mixture. Solid hydroxyl containing compounds are preferably milled and sieved prior to being mixed into the composition.
The adhesive compositions of the invention may be cured by exposure to a temperature profile used to reflow eutectic solder, namely ambient temperature ramped to 150xc2x0 C. at 90xc2x0 C./minute, held isothermal for approximately 1 minute, then ramped to about 220xc2x0 C.-250xc2x0 C. at 90xc2x0C./minute. A post-cure at 150xc2x0 C.-170xc2x0 C. for about 0.5 to about 2 hours may be used to complete the cure.
The adhesive compositions and resulting adhesives are useful to attach solder bumped flip-chips to a substrate and as an underfill adhesive or encapsulant for surface mounted components in general so to provide environmental protection for the surface mounted components. For example, the adhesive composition of the invention would be applied to the substrate, the chip placed onto the adhesive composition with solder bumps down, and then the component would be heated so to reflow the solder.